1. Technical Field
The present invention relates to an engine system control device to be applied to an engine system (system provided with an engine such as an automobile). The present Invention particularly relates to an engine system control device in the case where the engine has a variable compression ratio mechanism capable of changing a compression ratio.
2. Related Art
In an engine system, state determination of an intake and exhaust system, that is, failure diagnosis is performed by an engine electronic control unit (hereinafter, called as the “ECU”). Such on-board diagnosis (OBD) of the intake and exhaust system includes catalyst failure diagnosis, exhaust gas sensor failure diagnosis, and canister purge system failure diagnosis.
For example, in an exhaust passage for this type of system, a catalyst for purifying an exhaust gas is placed. This catalyst is deteriorated due to harmful components (such as lead and sulfur) in fuel and excessive heating. When the catalyst is deteriorated, an exhaust gas purifying ratio is degraded, and exhaust emission is increased. Thus, various devices for determining deterioration of the catalyst are conventionally proposed (such as Japanese Patent Application Publication No. 1993(H5)-133264 and 2004-28029).
As this kind of catalyst, a so-called ternary catalyst is widely used. This ternary catalyst has a function called an oxygen absorption function or an oxygen storage function. In the case where an air-fuel ratio of the fuel-mixing air is lean, such a function is to reduce NOx (nitrogen oxide) in the exhaust gas and absorb (store) oxygen taken from NOx inside. Meanwhile, in the case where the air-fuel ratio of the fuel-mixing air is rich, the function is to release the absorbed oxygen for oxidation of unburned components such as Him and CO in the exhaust gas.
Thus, the more a maximum value of an oxygen amount to be absorbed by the ternary catalyst (hereinafter, called as the “oxygen absorption amount”) is, the higher a purifying ability of the ternary catalyst is. Therefore, a deterioration state of the ternary catalyst can be determined by the maximum value of the oxygen absorption amount (hereinafter, called as the “maximum oxygen absorption amount”).
In a catalyst deterioration degree detection device disclosed in Japanese Patent Application Publication No. 1993(H5)-133264, a first air-fuel ratio sensor is arranged at a position on the upstream side of a ternary catalyst in an exhaust passage and a second air-fuel ratio sensor is arranged at a position on the downstream side of the ternary catalyst in the exhaust passage.
Deterioration determination of the ternary catalyst (calculation of the maximum oxygen absorption amount) with such a configuration is performed as follows. Firstly, the air-fuel ratio of the fuel-mixing air supplied into a cylinder of an engine is set to be a predetermined lean air-fuel ratio for a predetermined time. Thereby, oxygen is absorbed by the ternary catalyst up to a limit of an absorbing ability. After that, the air-fuel ratio of the fuel-mixing air is forcibly changed to a predetermined rich air-fuel ratio. Then, the air-fuel ratio detected by the second air-fuel ratio sensor is changed to the rich ratio after maintained to be a theoretical air-fuel ratio only for a fixed time Δt. The maximum oxygen absorption amount is determined based on a difference Δ (A/F) between the theoretical air-fuel ratio and the rich air-fuel ratio, the time Δt, and an intake air amount of this time.
This maximum oxygen absorption amount is changed even with a temperature of the ternary catalyst. Specifically, when the temperature of the ternary catalyst is increased, the maximum oxygen absorption amount is increased. Thus, when the deterioration determination of the catalyst is performed based on the maximum oxygen absorption amount calculated without considering the temperature of the catalyst, there is a problem that determination precision is not favorable. In a catalyst deterioration degree detection device disclosed in Japanese Patent Application Publication No. 2004-28029, the maximum oxygen absorption amount is corrected based on the temperature of the catalyst in a calculation period of the maximum oxygen absorption amount.
In order to control the air-fuel ratio of the engine, so-called air-fuel ratio feedback control is generally performed. This control is performed based on an output of an exhaust gas sensor (air-fuel ratio sensor) placed in an exhaust passage. In general, this exhaust gas sensor is an oxygen sensor for generating an output in accordance with oxygen concentration in the exhaust gas. This exhaust gas sensor is provided on the upstream side and/or the downstream side in the flowing direction of the exhaust gas relative to the catalyst for purifying the exhaust gas.
In general, as the exhaust gas sensor provided on the downstream side relative to the catalyst (second air-fuel ratio sensor described above), a solid electrolyte type oxygen sensor having an output characteristic in which an output is substantially fixed on the rich and lean sides of the theoretical air-fuel ratio whereas the output is radically changed around the theoretical air-fuel ratio is used. As the exhaust gas sensor provided on the upstream side relative to the catalyst (first air-fuel ratio sensor described above), the solid electrolyte type oxygen sensor or a limiting current type oxygen concentration sensor having a relatively linear output characteristic within a wide air-fuel ratio range is used.
When failure such as the deterioration is generated in the exhaust gas sensor, there is a fear that air-fuel ratio control of the engine is not properly performed. The exhaust gas sensor is also used to calculate the maximum oxygen absorption amount for the deterioration determination of the catalyst as described above. Therefore, when the failure such as the deterioration is generated in the exhaust gas sensor, the deterioration determination of the catalyst cannot be accurately performed.
Thus, a device for diagnosing the failure of this exhaust gas sensor is conventionally proposed (for example, refer to Japanese Patent Application Publication No. 2003-254135, 2004-225684, 2007-16712, and the like). This device is to determine whether or not the exhaust gas sensor normally works in accordance with a responding state of the exhaust gas sensor to an air-fuel ratio change of the fuel-mixing air. For example, in a device disclosed in Japanese Patent Application Publication No. 2004-225684, the air-fuel ratio is alternately forcibly changed between a predetermined rich air-fuel ratio and a predetermined lean air-fuel ratio, and presence of failure of a sensor is determined based on whether or not a sensor output rightly following this air-fuel ratio change is generated.